What is Huygens–Fresnel principle?

Example 22.5 (Eye, telescope, microscope)

The eye ($D = 3\,\mathrm{mm}$, $\lambda = 550\,\mathrm{nm}$): $1.22\lambda/D = 2.2 \times 10^{-4}\,\mathrm{rad}$, about $45''$ — a $1\,\mathrm{mm}$ detail at $4\,\mathrm{m}$, which is indeed about the acuity of a good eye: evolution matched the retina’s cells to the diffraction limit. A $2.4\,\mathrm{m}$ telescope: $0.06''$, a thousand times better, if the atmosphere lets it (it does not, from the ground: $1''$ of "seeing", whence space telescopes and adaptive optics). A radio dish of $100\,\mathrm{m}$ at $\lambda = 21\,\mathrm{cm}$: $9'$, worse than the eye; interferometers across continents restore the resolution. A microscope objective of aperture angle $\alpha$ resolves $0.61\lambda/n\sin\alpha$ — about $0.2\,\text{µ}\mathrm{m}$ in the visible, whatever the magnification: the limit that drove microscopy to electrons and to X-rays.